Don’t let that tree outside your kitchen window fool you. Though it may
not look like it’s doing much more than adding a little shade to your
yard, it is, in fact, working to curb global warming. Through the process of
photosynthesis, green plants use the energy from the sun to draw down carbon
dioxide and combine it with water to create the carbohydrates plants need to
grow and animals need to live. While a single tree does not have much effect on
the atmospheric levels of carbon dioxide, en masse plants draw down millions of
tons of this potent greenhouse gas and effectively help keep the Earth cool.

Given their positive effect on the climate, one would think that we’d
want to keep the world’s plants, especially those in our backyards,
intact. But each year humans destroy untold amounts of plants and fertile soil
through the process of urbanization. Every time a subdivision is built, a strip
mall is erected, or a road is laid, the local vegetation is uprooted and the
soil is turned. Though new grass and trees may sprout afterwards, this newly
grown canopy of vegetation is usually much less dense.

Urbanization tends to the reduce vegetation density. These two images show city lights and Normalized
Difference Vegetation Index (NDVI) for the eastern United States. Bright areas of the lights image are more urbanized. Dark green regions of the
vegetation image represent dense vegetation, while brown indicates sparse vegetation. Between the two images, urban areas correspond to regions with sparse vegetation.
(Lights image data courtesy Marc Imhoff of NASA GSFC and Christopher Elvidge of NOAA NGDC. NDVI data courtesy Compton Tucker and Bob Mahoney of
NASA GSFC.)

Tracking the precise impact of urbanization on vegetation, however, is no
simple task. Urbanization moves relatively fast and its outlines are often hard
to discern. It’s also difficult to gauge what the urbanized areas were
like before the cities were erected. Recently, a group of researchers at
NASA’s Goddard Space Flight Center led by biologist and remote sensing
specialist Marc Imhoff found a way to overcome these obstacles. Using satellite
images of city lights at night, they constructed a map of the urbanized areas of
the United States. They then retrieved vegetation density readings of present
day American cities as well as simulated readings of the landscapes that
pre-dated these cities. By combining the vegetation data with the urbanization
maps, Imhoff was able to calculate the effects of urbanization on many types of
ecosystems across the country.